The present invention relates to a method for anodizing silicon substrates and a method for manufacturing a surface-type acceleration sensor using the anodizing method.
Anodization of silicon substrates has conventionally been practiced in micro machining of silicon. FIG. 18 shows a conventional method of anodization. A silicon substrate 29 and a counter electrode 31 are immersed in an aqueous HF solution (aqueous hydrofluoric solution). The silicon substrate is an anode and the counter electrode 31 is made, for example, of Pt (platinum). An electric field is applied to the silicon substrate 29 and the counter electrode 31. The silicon substrate 29 includes a first portion, which will be porous, and a second portion, which excludes the first portion. The surface of the second portion of the silicon substrate 29 is covered with a resin film (protection film) 30 such as a photoresist for protection from the HF solution 27. During anodization, the first portion of the silicon substrate 29 becomes a porous silicon layer 25. In a later step, the layer 25 is removed by alkali etching to form a cavity in the silicon substrate 29.
However, since the resin film 30 does not closely contact the silicon substrate 29, the HF solution 27 may enter the space between the film 30 and the silicon substrate 29. This erodes the second portion of the silicon substrate 29.
To solve the above problem, a ceramic film, which has HF resistance, may be used instead of the resin film 30. However, the ceramic film is hard to form and is made by a different manufacturing process than an IC manufacturing process. This makes the ceramic film unsuitable for forming elements such as an acceleration sensor, the manufacturing process of which is close to the IC manufacturing process.
An objective of the present invention is to provide a method of anodization in which only a required portion of a silicon substrate is made porous.
Another objective of the present invention is to provide a method for manufacturing an improved surface-type accelerator sensor using the anodization method.
To achieve the above objectives, in the anodization of a silicon substrate of the present invention, the silicon substrate includes a first portion, which is made porous, and a second portion, which excludes the first portion. A metal protective film having HF resistance is formed on the surface of the second portion of the silicon substrate. During the formation of the metal protective film, a metal silicide having HF resistance is formed between the metal protective film and the silicon substrate. The silicon substrate covered with the metal protective film is immersed in the HF solution, a positive terminal of a direct current power source is connected to the silicon substrate, which serves as an anode, and a voltage is applied to the silicon substrate.
In the present invention, the metal protective film and the metal silicide are made of HF resistant metals, which include W (tungsten) and Mo (molybdenum). When the silicon substrate is immersed in and eroded by the HF solution during anodization, the portion covered with the metal protective film and the metal silicide are effectively protected. As a result, the surface of the silicon substrate covered by the metal protective film is not corroded by the HF solution.
In a method of the present invention for manufacturing a surface-type acceleration sensor, a first p-type silicon layer is formed on a predetermined area of a surface of a p-type single crystal silicon substrate by adding impurities. An epitaxial growth layer, which is made of n-type single crystal silicon, is formed on the upper surface of the p-type single crystal silicon substrate, such that the first p-type silicon layer is covered by the epitaxial growth layer. A second p-type silicon layer for forming an opening portion is formed in the epitaxial growth layer by adding impurities. A deformation gage, which is made of p-type silicon, is formed on the upper surface of the epitaxial growth layer. A wiring pattern, which is connected to the deformation gage, is then formed. A passivation film is formed to cover the wiring pattern with the second p-type silicon layer exposed. A metal protective film having HF resistance is formed on the surface of the passivation film and the epitaxial growth layer but not on the second p-type silicon layer. A metal silicide having HF resistance is formed between the metal protective film and the epitaxial growth layer. Anodization is performed to convert the first and second p-type silicon layers into porous silicon layers. The porous silicon layers are removed by alkali etching to form a beam, which is made of the epitaxial growth layer.